Unicef reported cases of boys and girls being raped, and young women being subjected to sexual demands in the slums in exchange for a promise of passage to Britain.

The plight of children trapped in the camps emerged in findings by Unicef France and Unicef UK as many seek to reach relatives in the UK after fleeing war, poverty and persecution in their homeland.

Researchers were told of examples of children being debt slaves and forced into helping people smugglers at ferry terminals.

Traffickers were now charging up to £5,500 per person to cross the English Channel to Kent - higher than ever before amid tightened security around the cross-Channel ports, the charity said.

The study examined the plight of 60 children aged from 11 to 17 from countries including Afghanistan, Eritrea and Iran, living in camps along the northern French coast between January and April.

Unicef said at most camps traffickers charged an "entry fee" before children were allowed to stay. Those unable to pay were forced into laborious tasks, such as selling food at a night market in the so-called Calais Jungle.

Some children have expressed a wish to be admitted to a psychiatric ward following mental health breakdowns amid the drudgery of life in the squalid camps.

Around 30 unaccompanied children at the Calais camp have so far been brought to the UK, Home Office minister Richard Harrington told the Commons this week.

And he said he expected the figure to increase "very significantly" as the Government attempted to speed up the family reunification process.

But the examples of the abuse being endured by some children in the camps had led Unicef to call for more rapid action by Britain.

Unicef UK deputy executive director Lily Caprani said: "The Prime Minister says unaccompanied children should be brought to the UK if they have family here, yet these children's cases are moving far too slowly.

"These camps are no place for a child. We know there are at least 157 children in Calais with the legal right to be with their family in the UK.

"The longer these children have to wait, the more desperate they may become and the more likely they are to risk their lives fleeing the appalling conditions of the camps to reach their families."

Unicef said there were 500 unaccompanied children at camps in Calais and Dunkirk as of March. The average stay was estimated to be five months, but the charity said some children had stayed for nine months.